1. Field of the Invention
The present invention relates to an optical amplifier for batch amplifying, wavelength division multiplexed signal light which includes a plurality of signal lights of different wavelengths, and to a wavelength division multiplexed light transmission system which transmits the wavelength division multiplexed signal light while amplifying and repeating. In particular the invention relates to an optical amplifier and a wavelength division multiplexed light transmission system which can effect gain equalization of wavelength division multiplexed signal light, and suppression of noise light.
2. Description of the Related Art
With conventional long distance optical transmission systems, optical transmission is performed using optical regeneration repeaters which electrically convert light signals to effect re-timing, re-shaping, and re-generating. However recently, with progress in the utilization of optical amplifiers, optical amplifier repeater transmission systems which use optical amplifiers as linear repeaters are being investigated. By replacing an optical regeneration repeater with an optical amplifier repeater, the number of parts in the repeater can be greatly reduced, with the prospect of maintaining reliability and greatly reducing costs.
Furthermore, as a method of realizing large capacity of an optical transmission system, a wavelength division multiplexed (WDM) light transmission systems which multiplexes and transmits a plurality of signal lights of different wavelengths on a single transmission path is attracting attention.
With a WDM optical amplifier repeater transmission system with a WDM light transmission system combined with an optical amplifier repeater transmission system, it is possible to batch amplify, signal light of various wavelengths using an optical amplifier, thus enabling the realization of large capacity and long distance transmission with a simple (economic) construction.
Presently, as a practical optical amplifier, there is for example the erbium doped optical fiber amplifier (referred to hereunder as EDFA). The basic construction of this, as shown in FIG. 10 comprises an erbium doped fiber 1 (referred to hereunder as an EDF 1), an excitation light source 2 for generating excitation light, and a multiplexer 3 for multiplexing light signals input to an input port IN with excitation light output from the excitation light source 2. The optical amplifier 100 of FIG. 10 has a forward excitation type construction with the propagation direction of both the signal light and the excitation light inside the EDF 1 in the same direction. As other basic constructions other than the forward excitation type there is the backward excitation type optical amplifier 100' as shown in FIG. 11, with the propagation directions of the signal light and the excitation light in opposite directions, and the bi-directional excitation type optical amplifier (not shown in the figure), with excitation light supplied from opposite ends of the EDF 1.
The gain-wavelength characteristic of an optical amplifier using such an erbium doped fiber is known to have two gain peaks as shown in FIG. 12. In FIG. 12 is shown one example of a gain-wavelength characteristic for when an EDFA is operated in a saturation region. With this gain-wavelength characteristic, the gain peak wavelength on the short wavelength side is in the vicinity of 1534 nm, while the gain peak wavelength on the long wavelength side is in the vicinity of 1558 nm. Moreover, with the wavelength bandwidth for where the gain-wavelength characteristic becomes comparatively flat (for example the 3 dB bandwidth or the 10 dB band width), that for the gain peak on the long wavelength side is wider than that for the gain peak on the short wavelength side. Therefore with a WDM optical amplifier repeater transmission system using an EDFA, it is common to position the wavelength division multiplexed signal light, with the gain peak wavelength neighborhood on the long wavelength side at the center.
Amplification of wavelength division multiplexed signal light is performed corresponding to a wavelength band where the optical amplifier gain-wavelength characteristic becomes comparatively flat as described above. However, in the case such as shown in FIG. 13 for example, where a plurality of optical amplifiers 100 are positioned between an optical transmitter Tx and an optical receiver Rx to perform multistage repeating, the gain differences for the signal lights of the respective wavelengths in the respective optical amplifiers 100 are accumulated. As a result, a large level difference is produced in the signal light of the respective wavelengths after multistage repeating, so that the optical receiver cannot normally receive the wavelength division multiplexed signal light.
Therefore, with conventional optical amplifiers, a gain equalizer for flattening the gain of the signal light of the respective wavelengths is provided to thereby prevent the above described accumulation of the gain differences. As an example, in the case where with an eight wave multiplexed light transmission system, being in a state of practical use, the channel spacing (wavelength spacing) of the wavelength division multiplexed signal light is 0.8 nm, then 5.6 nm is required for the signal light wavelength bandwidth. With this signal light wavelength band, it is known that even with transmissions of for example 10,000 km, gain equalization can be realized comparatively easily using just one type of gain equalizer (optical filter). Further, the channel spacing of 0.8 nm corresponds to a frequency spacing of 100 GHz in the 1.55 .mu.m band. This is the ITU international standard.
Moreover, with the conventional optical amplifier, it is common to provide, separate to the abovementioned gain equalizer, an optical filter dedicated to suppressing noise light (spontaneous emission light noise) in the gain peak wavelength neighborhood on the short wavelength side. That is, since the gain peak on the short wavelength side outside the band of the wavelength division multiplexed signal light generates an excess of noise light, then the optical SN ratio of the wavelength division multiplexed signal light deteriorates. The abovementioned optical filter is for preventing deterioration in the optical SN ratio by suppressing this excess of noise light.
However, with the abovementioned conventional optical amplifier, since the two optical devices, namely the gain equalizer and the filter for suppressing noise light are provided separately, there is the problem that the overall construction of the optical amplifier becomes complicated, and the need also arises for compensating for insertion loss and polarization dependence loss generated by each of the two optical devices, so that there is an increase in the cost of the optical amplifier.
In particular, in realizing a WDM optical amplifier repeater transmission system aimed at even greater capacity transmission, the abovementioned gain equalization of the wavelength division multiplexed signal light and noise light suppression become important.
For example, considering the case of a sixteen-channel WDM light transmission system, when the channel width is made 0.8 nm, then 12 nm becomes necessary for the signal light wavelength bandwidth of the optical amplifier. To realize an optical amplifier having such a wide signal light wavelength band width, then in the gain peak wavelength neighborhood on the long wavelength side, the wavelength bandwidth where the gain-wavelength characteristic becomes flat must be widened. For this, there is known a method where for example as shown in FIG. 14, the optical input power to the optical amplifier is reduced so that the optical amplifier is operated in the unsaturated region. FIG. 15 shows an example of the gain-wavelength characteristic for when an EDFA is operated in the unsaturated region.
When as in FIG. 15, the EDFA is used in the unsaturated region, the bandwidth of the gain flattened part in the gain peak wavelength neighborhood on the long wavelength side becomes wide. However, the gain in the gain peak wavelength neighborhood on the short wavelength side increases, and hence an excess of noise light is further generated. Therefore, compared to the case where the EDFA is operated in the saturated region, there is the likelihood of a significant degradation of the optical SN ratio of the wavelength division multiplexed signal light. Consequently, the requirement for gain equalization of the wavelength division multiplexed signal light, and for noise light suppression is increased.